Caving Cobbles and Lost Circulation
A driller is bidding a 12-inch borehole for a 6-inch community well in a glacial outwash valley. Logs show 0-25 ft sandy loam, 25-85 ft clean sand and gravel, 85-130 ft coarse gravel with cobbles, and shale below 130 ft. Static water is 55 ft; target depth is 180 ft. The driller starts with mud rotary, but at 100 ft the hole begins caving and drilling-fluid returns drop sharply; the borehole is losing mud to the coarse gravel. The proposed completion is 6-inch Schedule 40 PVC casing to 120 ft, a 40-slot screen from 120 to 180 ft, and a gravel pack. The owner wants a pump test and a sanitary seal.
1. What drilling method and fluid program would you use to get through the caving cobble zone, and why?
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Use mud rotary with a well-conditioned bentonite or polymer slurry and drill through the cobble zone under temporary surface casing, or switch to cable tool if cobbles make rotary penetration impractical. The reason is that the hole is caving and losing returns in coarse gravel; cable tool can case as it goes and handles cobbles, while rotary needs filter-cake control. If staying with rotary, set 10- or 12-inch temporary steel casing through the caving zone to about 130 ft, maintain mud viscosity and gel strength, and use lost-circulation material such as mica, nut hulls, or a bentonite blend rather than simply adding water. Do not use air rotary in a caving, water-bearing sand and gravel without casing; it can worsen collapse and drive fines into the formation. The objective is to keep the borehole open long enough to set permanent casing and place a continuous annular seal.
2. Evaluate the proposed screen and gravel pack. What information do you need before selecting slot size?
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A 40-slot screen has 0.040-inch openings, which is likely too coarse for clean sand and gravel; it will pass a large fraction of formation fines and produce a sand-producing well. Before selecting slot size, obtain sieve analyses of formation samples from 120-180 ft. Design the gravel pack around the formation gradation, and select the screen slot so the screen retains most of the pack while the pack retains the formation. A common filter-pack approach is to use a clean, rounded pack material graded coarser than the formation, with a pack thickness of roughly 3-5 inches around the screen. The borehole must be large enough to accommodate the screen, pack, and a grout seal above. If the formation is fine, a finer slot and graded filter pack are needed; 40-slot may be appropriate only if the formation is coarse. The driller should not finalize the screen before the sieve analysis.
3. What does the lost-circulation zone imply for the sanitary seal, and what should be done before the pump test?
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Lost circulation means the borehole is connected to highly permeable gravel, so the drilling fluid is escaping and a good filter cake may not form. The same zone can take grout, leaving an incomplete annular seal and a pathway for surface contamination. Before completion, the driller should case off the loss zone, restore returns, and plan for staged or thicker grouting if losses continue. Verify grout returns at the surface and use a positive-displacement tremie. The pump test should be run only after the well is developed, sand production is controlled, and the annular seal is verified. If the seal cannot be placed reliably, the well should not be accepted as a public-health-protective installation.